Wave length modulation system



Aug. 2v2, 1944. J. FINCH WAVE LENGTH MODULATION SYSTEM Filed April 25, w41

4 Sheets-Sheet l Aug. 22, 1944.v J, F|NH 2,356,390

- WAVE LENGTH MODULATION SYSTEM Filed April 25, 19111 4 Sheets-Sheet 2V INVENTORy l BY J [.fwlp

A1'roRNEYv 4 Sheets-Sheet E J. L. FINCH Filed April 25, 1941 WAVE LENGTH MODULATION SYSTEM AUORNEY Aug. 22, '1944.

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J. l.. FlNcH WAVE LENGTH MODULATION SYSTEM Filed April 25, 1941 4 sheet-s-sheet 4 v INVENTOR Y @fa/nef Patented Aug. 22, 1944 2,356,390 Y WAVE LENGTH MoDULATIoN s-YsrEM James Leslie Finch, East Rockaway, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 25, 1941, Serial No. 390,228l

13 Claims.

This application concerns a new and improved method of and means for wave length modulation and is particularly concerned with spaced wave signalling systems wherein wave energy is keyed or shifted from one frequency (which may be called the marking frequency) to another frequency (which may be called the spacing frequency).

For the transmission of telegraph signals, including time division multiplex( and for teletype printers and for photoradio services using the CFVD principle, which in the past have generally used on and off keying, tests have been made recently using frequency modulation. In this type of transmission the marking characters, corresponding to the power on condition for the on and off system, are constituted by power at one radio frequency and the power off condition by power at a different radio frequency. This type of transmission is also referred to as spacing wave keying."

With this type of transmission it is particularly important that the receiver be kept tuned accurately to the transmitter signal. The necessary accuracy may be greater than that which is commercially practical to maintain in the transmitter frequency and in the adjustment of the receiver oscillators and circuits. For on and off keying automatic means have been employed for keeping the receiver in step with the trans-A mitter. These methods in general have drawbacks when spacing wave keying is used.

It is the purpose of my invention to provide means for the automatic adjustment oy the receiver which will work satisfactorily with the spacing wave type of keying.

At the transmitter I make use of an accurate frequency control device for holding both frequencies, i. e. the spacing and marking frequencies, as nearly constant as commercially practical. I provide means for determining the difference between the marking wave frequency and the spacing wave frequency much more accurately as measured in cycles per second than the accuracy of either the marking orthe spacing frequencies themselves.

At the receiver I pass both the marking wave and the spacing wave and their important side frequencies through a filter. This filter eliminates noise and interference which does not lie within its pass band. Arrangements are made such that the total power received in the pass band, which includes marking Wave power, spacing wave power and side frequency power, will operatelan automatic gain control soV as to keep this power nearly constant. This automatic gain control is made so rapid that amplitude variations due to the keying characters are substantially eliminated.

The resulting signal power is now passed through a system of filters including a high pass filter with its cutoff point somewhat above the higher of the two transmitter frequencies, and a low pass filter with its cutoff point somewhat below the lower of the two transmitter frequencies. Both of these cutoffA points are within the pass band of the first mentioned filter. During high speed keying some of the side frequengcies will be passed by the high pass and low pass filters. When the receiver is centered properly the amounts passed by the two filters will be substantially equal.

Should the transmitter frequencies rise the high pass filter will pass more power and the low pass filter will pass less. This unbalance will put into operation a corrective means which will readjust the receiver circuits so as to affect a balance.

I am not familiar with any device for this purpose in the prior art. Accordingly it might be said that this invention eliminates the necessity for frequent manual adjustments which would otherwise be required in the equipment.

In describing my invention reference will be made to the attached drawings wherein;

Figure 1 shows a system adapted to receive,

' amplify and record wave energy of the spaced wave type and includes novel means for maintaining the receiver adjustment in synchronism with the transmitted wave.

Figure 2 comprises curves and graphs illustrating the nature of certain filters used in the receiver circuit; while Figure 3 is a modification of the arrangement of Figure 1.

Figure 4 shows the characteristics of the high and low pass filters of the units il and l2 of Figure 1.

Figure 5 shows the characteristics of the high and low pass filters of the units 23 and 24 of Figure 1. f

Figure 6 shows the characteristics of the outputs of the filters and 36 as supplied to the rectifier 39 of Figure 3.

Figure 7 illustrates the driving means for the tuning element in the receiver of Figures 1 and 3.

A specific embodiment of this invention is shownin Figure 1. Typical receiver circuits are used in this system andunits thereof and their I connections are shown by block diagram.. I is a receiving antenna coupled to a radio frequency amplifier 2, which feeds the amplified wave energy to a detector 4 associated with an oscillator 3 for heterodyning the incoming signal to a lower frequency, l is the first detector, and its output is coupled to an intermediate frequency amplifier and band pass filter. The intermediate frequency energy passed by the filter 5 is fed to a frequency modulation detector system 6. 'l is a recorder or other translation means coupled with the output of detector 6 whereby the int'elligence can be transcribed into written form. This recorder may be located at a remote central ofiice, in which case suitable means for operating it remotely will be provided.

The circuits and apparatus in unit 6 comprise a frequency modulation detector system. For illustrative purposes I have shown it to include a second oscillator 8 and a second detector 9. This second oscillator is assumed to be set at such a frequency that the output of the second detector 9 will be an audio frequency. This audio frequency will vary in frequency in accordance with the incoming radio signals.- I-t is amplified by audio frequency amplifier l and is impressed on high pass filter l l and on low pass filter l2. High pass lter H is so arranged and adjusted that it passes currents of a strength proportional to the frequency of the audio wavesover a band of frequencies including the marking frequency, the spacing frequency and the essential side frequencies. Similarly, low pass lter l2 passes currents of a strength inversely proportional to the frequency of the audio waves. The characteristics of these high and low pass lters are shown respectively by the curves Il and I2? of Figure 4 of the drawings. The currents from Vthe two filters go to third detectors I3' and |4- respectively. The outputs of these detectors 'are impressed on combining unit I differentially. The current in the output of this combining unit will now be amplitude modulated essentially in accordance with the original signals at the transmitter. This current will now be suitable for operating a recorder directly or for keying a tone keyer for transferring the signals over a tone channel or for any other useful purpose. v

A fast automatic gain control deviceV lreceives power from the output of the intermediate frequency amplifier 5. i6 then reacts on the gain controls of the radio frequency amplifier 2 and the intermediate frequency-amplifier 5 so as -to maintain the amplitude of the output of the latter substantially constant.

In unit I1 is a system of circuits arranged to keep the receiver in tune with the incoming signals. A preferred embodiment of this device is illustrated. In this embodiment power from the intermediate frequency amplifier 5 is impressed on high pass filter 23 and low pass filter 24. High pass filter 23 has its cutoff slightly above the higher of the incoming marking and spacing frequencies while the low pass filter has its cutoff` slightly lower than the lower vof these two frequencies. These filter characteristicsare shown at 23' and 24 in Figure 5. The intermediatefrequency circuit characteristic is shown at 5. The outputs from these two filters are impressed upon rectifiers 25 and 26 respectively. Theoutputs of these two rectifiers operate differentially a frequency control device 21 which reacts on the frequency of the first oscillator'. Y2l maybe any suitable device such forinstance as a reversi ble motor which is geared toidrive a Vernier condenser in the first oscillator. Fora similar arrangement, see Usselman Patent #1,794,932, dated March 3, 1931. I8 is a cutoff device disposed between 21 and 3 and so arranged that the driving means in 21 cannot influence 3 should the signals be so weak that the automatic gain control is not capable of maintaining full output from the intermediate-frequency amplifier 5. Various means can be used for the disruption of the control of the oscillator by the rectifier output. For example, I use an electric clutch of the type shown in Ranger Patent #1,770,493, Artzt v #2,176,949 or Long #1,706,032 interposed between the motor of the said Usselman patent and the movable element of the reactance in the oscillator circuit. The gain control currents used directly or amplified excite the clutch winding to interrupt the drive in the presence of weak signals.

The operation of this device can be understood by reference to the circuits of Figure 1 and by reference to the curves of Figures 2, 4 and 5. In Figure 2, the abscissae of all the curves represent radio frequencies. These curves are plotted to an arbitrary scale. As the abscissae increase the frequency is assumed to increase. The ordinates of these curves are plotted to represent the strength of the component frequencies in decibels below the strength of the main radio frequency wave when it is held constantly on either the marking or the spacing frequency. Abscissa S corresponds to the spacing wave frequency, M corresponds to the marking wave frequency, C corresponds to the lower limit of the essential side frequencies and D corresponds to the upper limit of the essential side frequencies. The curves in Group I represent the spectrum of frequencies generated in frequency modulation service when short dots are being transmitted. Specifically this group indicates the frequency spectrum when the dots are recurring at a frequency corresponding to 1/20 of the frequency difference between the marking and the spacing waves. Thus if the marking wave is, for instance, 15,002,000 cycles and the spacing wave is 15,000,000 cycles the dot frequency would be 100 cycles.

These curves correspond to square wave keying with the marking characters corresponding to' 10% and the spacing characters corresponding to of the duration of one complete cycle. Actually under these conditions the side frequencies would theoretically extend to infinity. In practice it would not be necessary to transmit the side frequencies outside of the band C-D. It will be noted that this group of curves represents two sets of curves, one symmetrically about frequency S and the other symmetrical about frequency M. The lines drawn correspond to the envelope of the peaks of the waves at the various frequencies. Actually the frequencies in one group will combine with adjacent frequencies in the other group so that in general their powers will add. It should be noted that the two sets of curves are identical except for the frequencies at their centers, i. e. at the spacing frequency the level is -1 db. while at the marking frequency the level is -20 db. but in all other cases the two are equal.

The curves of group II are plotted in a manner similar to those of group I except they correspond to 90% mark 10% space signals. This group of curves is identical to group I except that 'in this case the level on the spacing frequency is 20 db. and that on the marking frequency is -1 db.

When thesignalsareof a Weight of 50% mark and 50% space the strengths will be represented by group III. In this case only the odd harmonics of the keying frequency are represented in the frequency spectrum.

' Curve 5 of Figure 5 represents the band pass characteristics of the IF amplifier 5. Curve 23 of Figure represents the pass characteristics of high pass filter 23 and curve 24' represents the pass characteristics of low pass lter 24. Curve l'l of Figure 4 represents the pass characteristics of high pass filter l I and the curve I2 represents the pass characteristics of low pass filter l2. In all of these filter curves, the ordinates represent voltage passed by the filters, while frequency is represented by abscissae, it being assumed that the input voltage in each case is held constant and that the frequencyis varied. y

An inspection of the curves of Figures Zand 5 will reveal that the power in the side frequencies between C and A and between B and D remains essentially constant regardless of the type of keying, provided of course the character repetition frequency remains constant. Thus when the receiver is properly centered the output of filter 23 and that of 24 will remain balanced. Should the transmitter frequency drift the resulting unbalance will correct the first oscillator frequency and recenter it. This centering effect'will be the same regardless of the type of signals being transmitted.

In operation the setting of the filters 23 and 24 corresponds to the highest keying frequency to be used. When it is desired to use a lower keying frequency then appropriate adjustments can be made to decrease the pass band of 5 and to locate the cutoff points A and B closer to the spacing and marking points S and M. However, should these lters remain correct for the highest frequency of keying and a lower frequency is employed the system still operates satisfactorily. 1f we assume for instance that a very low keying frequency is employed the side frequency power -will be concentrated very close to the spacing and marking frequencies and when the receiver is centered none will be passed by filters 23 and 24. Now should the transmitter frequency drift Fthere will be no corrective effect until the spacing `frequency tends to drift past point A or the marking frequency tends to drift past point B of VFigure 2. As soon as this happens the corrective devices operate to prevent further drift. This maintains the receiver adjustment suiciently accurate for transmitting this low speed keying. Now should high speed keying be resumed the receiver will automatically -be returned as described above.

It will be understood that the transmitter is keyed continuously in order that the automatic features at the receiver remain in operation. This keying may be at a low speed during standby periods or it may be maintained at-the traffic 'speed which is being employed at the time.

The system described above can be employed in connection with a space diversity receiving system as is the present practice with on and off keying. In this case a common automatic gain lcontrol is used so as to maintain the total output 'from the diverse receiving systems constant. "This tends to favor the particular receiving lsystem which at a particular time is picking up 4`the strongest signal.

'f Further, this system can be applied to any' :other type of diversity system such as, for inresponding to a different vertical angle of arrival of the received signals.

The frequency modulated wave demodulator 6 may be as described above or may take the form of any known frequency demodulator such, for instance, as that described in an article by Mathes and Whitaker on Radio Facsimile by Subcarrier Frequency Modulation in the October 1939 RCA Review.

Another specific embodiment of my invention is shown in Figure 3. In this figure parts corresponding to those in Figure 1 are assigned the same numbers. In this modification relay devices have relay windings 31 and 38 connected to the output of the combining circuits in unit l5 and the relays are keyed in accordance with the received signal. These relay devices each have two armatures 4l and 43 and 42 and 44 which operate to connect a marking frequency filter 35 into a circuit during the time when marking frequency signals are being received. Thus, during intervals when the receiver is picking up the marking frequency the armatures are moved to a position (up) completing a connection from the output of the audio amplifier I0 through lines 45, armatures 4l and 43, lines 41, the marking frequency filter 35, lines 49, armatures 42 and 44 and lines 5l to the detector in 39. Alternately these relays connect into circuit the spacing frequency filter 38 during each interval when a spacing frequency character is being received. This latter circuit is completed through lines 45,

armatures 4I and 43, lines 53, lter 3B, lines 55,`

armatures 42 and 44 and lines 5I. The full wave rectifier 39 supplies current to a motor in 21 of the two-wire type operating against a spring. This motor may comprise a solenoid operating against a spring and driving a tuning condenser by way of a cut-out device I8 as described above. The detector 39 supplies output current to this motor or solenoid winding. In Figure 7 I have shown a motor and circuit arrangement to be used in this automatic control circuit. The lines 5| supply intermediate frequency or tone representing mark or space depending on the position of relay contacts 4l, 43 and 42, 44 to the full wave rectifier 39. The direct current component in the output of 39 is supplied to an actuating winding in motor M, which drives the rotor plates of the tuning condenser, which may be included in the circuit of oscillator 3. The motor works against a drag, such as, a spring, and through, for example, an electrical clutch, the opening and closing of which depends on the strength of the fast automatic gain control potentials out of I6. This clutch may be included in the cut out device at I8.

Filters 35 and 36 are arranged so that in combination with detector 39 they will maintain the receiver in proper adjustment for both the marking and the spacing characters. For instance, filter 35 may be a low pass lter with the middle of its cutoff characteristic set at the normal audiofrequency Maf corresponding to the marking frequency characteristics as illustrated by curve 35 in Figure 6. This iilter output will be detected 4by 39 so as to hold the required first oscillator frequency. Now should anything change to cause this audo frequency to rise a smaller amount of power will be delivered to the rectifier 39 and this will operate 21 so as t0 change the first oscillator frequency to correct the frequency at the output of l0. When a twowire motor or solenoid as described above is used -"stance, one employing twoor more receivers each the current through the motor winding decreases permitting the spring to relax to thereby drive the tuning element of the oscillator. Similarly Se is a low pass filter having a characteristic as shown at 36 of Figure 6 but with the frequency at the middle of its cutoff corresponding to the audio frequency Scf for the spacing wave and its action on the first oscillator will be the same as that of 3E.

Obvious variations of Figure 3 can include changing the audio frequency derived at the output of unit d to a super-audio frequency or the circuits can be rearranged so that the intermediate frequency from the output of unit 5 can be used directly on filters and 35. The relay devices 3l and 38 may be electric switching devices, That is, these switching means may be of the electronic type wherein a tube stage is biased to pass or cut off current in accordance with the output of the combining unit in l5. in this arrangement the switching tube inputs are connected to the output of ll and their outputs connected to line .il and their ability to relay or amplify voltage controlled by the output of I5. Electronic switcliers are well known in the art and need no further description here.

- In operation the motor control may be manual until the circuit is lined up properly and then the automatic control is switched in.

It should be understood in the above discussions that when reference is made to high and low pass filters and increasing and decreasing frequency of the incoming signal account must be taken of the possible inverting effect should the first oscillator frequency be set at a higher value than that of the incoming signals instead of being set at a lower value as assumed in the above description.

I claim:

l. The method of maintaining a receiver of wave length modulated signals, comprising carrier energy the frequency of which is keyed alternately from a first frequency to a second frequency and vice versa, and the side bands produced by said keying, tuned to the said signals which includes these steps, receiving the said energy, filtering the received energy to remove therefrom noise components and to select therefrom energy within a band of frequencies including said carrier energy at said first and second frequencies and the upper and lower side bands of the carrier energy in its keyed positions in the frequency spectrum, amplifying the filtered and selected energy including the modulation side frequency power and controlling the tuning of the said receiver substantially only in accordance with the side frequency power.

2. The method of maintaining a tunable receiver of constant frequency, variable dot telegraphy signals, represented by radio power including a carrier of a first frequency for marking and the side bands generated by its interruption and radio power including a carrier of a. second frequency for spacing and the side bands generated by its interruption, tuned to substantially center on a frequency intermediate said rst and second frequencies which includes these steps, selectively relaying modulation side frequencies only within the spectrum where they are unaffected by variation in dot length, separately rectifying the selectively relayed side frequencies and opposing the rectified currents to derive a resulting current for tuning adjustments.

3. The method of maintaining a tunable receiver of telegraphy signals, comprising radio energy keyed by signals from a first frequency to a second frequency 'to provide in effect two cur- 75 rents of different frequencies and their side fre-- quencies, tuned to substantially center on the mean of said first and second frequencies which includes these steps, receiving the said signals, amplifying the said signals, controlling the amplification of the signals at a fast rate to remove any variations in signal strength and to keep the power thereof nearly constant, selecting side frequency energy in the frequency spectrum above the frequencies of said rst and second frequencies, selecting side frequency energy in the frequency spectrum below the frequencies'of said first and second frequencies, separately rectifying the selected side frequencies, balancing the energy derived by rectifying one side frequency against the energy derived by rectifying the other side frequency and controlling the tuning of said receiver by the power not balanced out in the preceding step.

4. The method of maintaining a tunable receiver of wave length modulation constant frequency, variable dot telegraphy signals represented by radio power of one frequency for marking and radio power of another frequency for spacing, tuned to substantially center on the mean of two said frequencies which includes these steps, limiting the amplitude of the signals at a rate to hold the same of nearly constant power, selecting the upper side bands falling above the highest of said two frequencies, selecting the lower side bands falling below the lowest of said two frequencies, opposing the selected upper side band pOwer against the selected lower side band power and utilizing the resultant power for tuning the receiver.

5. A tunable receiver for amplifying and demodulating wave energy comprising alternately keyed carriers of a first and second frequency, a high pass filter having its cutoff below the lowest of said two frequencies, a low pass filter having its cutoff above the highest of said two frequencies, connections for impressing said wave energy on said filters, a detector coupled to each filter, connections for combining the outputs of said detectors differentially, and means for main.. taining the receiver in tune, including a low pass filter having its cutoff frequency below the frequency of the lowest of said carriers, a high pass filter having its cutoff frequency above the frequency of the highest of said carriers, connections for impressing said wave energy on said last two filters, rectifiers for rectifying the outputs of said last two filters, and connections for controlling the receiver adjustment in accordance with the rectified outputs.

6. A tunable receiver for amplifying and demodulating wave energy comprising alternately keyed currents of a first and second frequency, a filter having a sloping frequency versus ampltude characteristic over a band of frequencies including said two frequencies, a second filter having an oppositely sloping frequency versus amplitude characteristic over a band of frequencies including said two frequencies, connections for impresing said wave energy on said filters, a detector coupled to each filter, connections for combining the outputs of said detectors differentialiy, and means for maintaining the receiver properly tuned including a low pass filter having its cutoff frequency above the keying frequency of one of said carriers, a low pass filter having its cutoff frequency above the keying frequency of the other of said carriers, means for alternately impressing wave energy on said filters, means for alternately connecting a rectifier to the output of said filters, a control circuit for controlling the tuning adjustment of the receiver in accordance with the output of said last rectifying means.

7. In a radio communication system using signals at one frequency to give one signalling condition and of a different frequency to give an alternate signalling condition, a tunable receiver including means for demodulating said signals and recording the same, means including a high pass filter which passes only frequencies higher than both of the said two frequencies and a low pass filter passing only frequencies lower than both of the two said frequencies, said filters having cutoffs spaced outside the band determined by the two signal frequencies by an amount such that at the highest normal signalling speed the side frequencies beyond the cutoffs will remain substantially equal in strength regardless of relative duration of the two signalling conditions, and means for combining the outputs of said filters to control the receiver tuning adjustment.

8. In a radio communication system using modulation signals at one frequency to give one signalling condition and of a different frequency to give an alternate signalling condition, tunable means for translating said signals and means for demodulating the translated signals and recording the same, a low pass filter having a cutoff above the modulation frequency on one of said signals, a low pass filter having a cutoff above the modulation frequency on the other of said signals, two relays, each including a winding and contacts, with the windings of both relays coupled to said demodulating means to be excited thereby, a circuit including the contacts of one of said relays for coupling a selected one of said filters to said signal translating means, a rectifier, connections including the contacts of the other of said relays for coupling the selected one of said filters to said rectifier, a control for said tunable means and a coupling between said rectifier and said control.

9. In a radio communication system using signals at one frequency to give one signalling condition and of a different frequency to give an alternate signalling condition, tunable means for amplifying and demodulating said signals and recording the same, means including a high pass lter which passes only frequencies higher than both of the said two frequencies and a low pass filter passing only frequencies lower than both of the two said frequencies, said filters having cutoffs spaced outside the band determined by the two signal frequencies by an amount such that at the highest normal signalling speed the side frequencies beyond the cutoffs will remain substantially equal in strength regardless of relative duration of the two signalling conditions, and means for combining the outputs of said filters to control the tunable means.

10. In a tunable receiver of wave energy cornprising a carrier, the frequency of which alternates in accordance with signals from a first frequency to a second frequency, and its side frequencies, the first and second frequencies and rate of alteration being such that the side frequencies overlap and their powers add, a filter having a pass characteristic which passes said carrier power and side frequency power said filter having an input on which said wave energy is impressed and an output from which the power passed may be utilized, a rectifier coupled to the output of said filter, a fast gain control circuit in said receiver actuated by the output of said rectifier to keep the power out of said filter nearly constant, a high pass filter having its cut-off frequency above the frequency of the highest of said two frequencies, a low pass filter having its cut-off frequency below the frequency of the lowest of said two frequencies, means for impressing power passed by said first filter on said high and low pass filters, a rectifier system coupled with said high and low pass filters, and connections for tuning said tunable receiver in accordance with the output of said rectifier system.

ll. In a system for demodulating wave energy keyed in accordance with signals from a first frequency to a second frequency spaced from said first frequency by an amount such that the side band frequencies caused by said keying overlap, a tunable wave energy receiving means, an amplifier of variable gain coupled to the wave energy receiving means, a band pass filter, the pass of which is wide enough to pass said first and second frequencies and the side frequencies, excited by said signals, said band pass filter having an output, a rectifier for deriving a potential depending upon the amplitude of the signals coupled with the output of said band pass filter, connections from said rectifier to said amplifier for fast control of the amplitude of the signals impressed on the input of said band pass filter,

a lter which passes only frequencies higher than either of the said two frequencies, a filter which passes only frequencies lower than either of the said two frequencies, a rectifier coupled to each of said last two mentioned filters, a circuit for opposing the outputs of said last mentioned rectifiers to produce a resultant which is substantially Zero when the receiver is properly tuned and deviates from zero in a direction depending on the direction of detuning of the receiver, and connections from said last named circuit to the tunable receiving means for retuning the same in accordance with said resultant.

12. A system as recited in claim 11, including means connected to said first named rectifier and actuated by the output thereof for inhibiting operation of said tuning means on the reception of wave energy below usable strength.

13. In a radio communication system using signals at one frequency to give one signalling condition and of a different frequency to give an alternate signalling condition, a tunable receiver including means for demodulating said signals and recording the same, a lter which passes only frequencies higher than either of said two frequencies and a filter which passes only frequencies lower than either of the two said frequencies, said filters rejecting a band of frequencies between said higher and lower frequencies passed which rejected band of frequencies include the said two signal frequencies, connections for impressing said signals on said filters, a rectifier coupled to each of said filters, and connections for combining the outputs of the rectifiers and controlling the receiver tuning adjustment in accordance with the combined output of the rectifiers.

JAMES LESLIE FINCH. 

